<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div  class="div">
    <div class="left">左侧</div>
    <div class="center">main</div>
    <div class="right">右侧</div>
  </div>
</body>
<style>
  .div {
    display: flex;
    justify-content: space-between;
    width: 500px;
    height: 300px;
  }
  .left {
    width: 100px;
    height: 100%;
    background-color: aqua;
  }
  .right {
    width: 100px;
    height: 100%;
    background-color: blue;
  }
  .center {
    /* width: 100%; */
    height: 100%;
    background-color: brown;
  }
</style>
</html>